In drilling operations the cutting teeth (buttons) on the drill bits or cutters become flattened (worn) after continued use. Regular maintenance of the drill bit or cutter by regrinding (sharpening) the buttons to restore them to substantially their original profile enhances the bit/cutter life, speeds up drilling and reduces drilling costs. Regrinding should be undertaken when the wear of the buttons is optimally one third to a maximum of one-half the button diameter.
Manufacturers have developed a range of different manual and semi-automatic grinding machines including hand held grinders, single arm and double arm self centering grinding machines and grinders designed specifically for mounting on drill rigs, service vehicles or set up in the shop. The present invention is particularly applicable to mobile grinding apparatus of the type described in U.S. Pat. No. 5,193,312 and semi-automatic grinding machines as described in U.S. Pat. No. 5,070,654 and in International Application published under WO 02/04169.
These types of machines utilize a grinding machine having a spindle or rotor rotated at high speed, typically about 12,000 to 22,000 RPM. A grinding cup mounted on the end of the rotor or spindle grinds the button and typically the face of the bit/cutter surrounding the base of the button to restore the button to substantially its original profile for effective drilling. In addition to the rotation of the grinding cup, these types of grinding machines may include features where the grinding machine is mounted at an angle to the longitudinal axis of the button and the grinding machine is rotated to provide orbital motion with the center of rotation lying in the center of the grinding cup. When grinding the buttons, the centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button. On buttons where wear is uneven, typically gauge buttons, this may result in regrinding the button off center from its longitudinal axis.
The conventional grinding machines switch between grinding pressure and balance pressure to achieve the desired effect. This, for example, does not allow for a grinding pressure equal to zero. In conventional grinding machines, the minimum grinding pressure is equivalent to the weight of the arm or lever section and the components attached to it.
Longstanding problems with these types of grinding machines are vibration and noise due to high rotational speeds, wear, the requirement for large compressors for pneumatic systems and long grinding times per button, in the larger sizes of six minutes or more.